For centuries the solar system has provided an important test bed for new concepts and theories. For example, the anomalous precession of Mercury's orbit was the first successful test of Einstein's theory of general relativity. Today, the high precision of modern instruments makes it possible to probe ever more minute astronomical effects. The tacit hope is that one day astronomers will detect some slight departure from the predictions of general relativity.
This is the underlying tenor of recent work by John Anderson of the California Institute of Technology and colleagues (Phys. Rev. Lett. 81 2858). They report an intriguing anomaly in the recorded motion of three spacecraft: Pioneer 10, Pioneer 11 and Ulysses. Pioneer 10 and 11 are moving out of the solar system after completing their main missions - to explore the outer planets - while Ulysses is on an elongated orbit that roughly bridges the orbits of Jupiter and Earth.
The motion of these spacecraft is governed by the gravitational fields of the known bodies in the solar system, and can be calculated very accurately from general relativity. Anderson's analysis shows a small but systematic departure from the expected motion. Indeed, the spacecraft move as if they were subject to a new, unknown force pointing towards the Sun. This force imparts the same constant acceleration, ap, of about 10-7 cm s-2 to all three spacecraft, about ten orders of magnitude less than the free-fall acceleration on Earth. Such a finding, if it were not explained away by some mundane effect, would be a major break with accepted physics.
Mordehai Milgrom in the Department of Condensed Matter Physics, Weizmann Institute, Israel, discusses the implications of this anomaly in the January issue of Physics World magazine.